The present invention relates to an antenna device and a reception system for receiving radio waves of digital radio broadcasting.
In recent years, a digital radio receiver which receives a satellite wave or a terrestrial wave and provides digital radio broadcasting has been developed, and has been put to practical use in the United States. The digital radio receiver is mounted in a mobile station such as a vehicle so that radio waves in a frequency band of about 2.3 GHz are received, thus making it possible to provide radio broadcasting to the vehicle, for example. That is, the digital radio receiver is a radio receiver which can provide mobile broadcasting. Since frequencies of the received radio waves are in a frequency band of about 2.3 GHz, the received wavelength (resonance wavelength) ??at that time is about 128.3 mm. Further, the terrestrial wave stems from the satellite wave being received in an earth station and then the frequency of the wave being a little shifted and the wave being re-transmitted in a form of a linearly polarized wave.
In general, when the digital radio receiver is mounted in the vehicle, the antenna device is attached on the roof of the vehicle. As shown in FIG. 1, the antenna device 10 (for example, see Japanese Patent Publication No. 2005-109687, paragraph 0016 to 0022, FIG. 1) for such the digital radio receiver includes an antenna case 13 made by coupling a top cover 11 with a bottom plate 12, an antenna module 14 disposed in the top cover 11, a packing member 15 disposed in a junction portion of the top cover 11 and the bottom plate 12 and ensuring a sealing-ability of the antenna case 13, and a signal line 16 connected to the antenna module 15.
The antenna module 14 has an antenna element 20 in which an antenna receiving a signal transmitted from a satellite is formed and a circuit board 21 in which a circuit for performing a signal processing such as amplification of the signal received by the antenna element 20 is formed. The antenna element 20 and the circuit board 21 are joined each other by a two-sided tape 22 or the like.
A coaxial cable, as a signal line 16, for fetching signals outside from the antenna case 13 is connected to the circuit board 21. Further, in the circuit board 21, a shield case 24 for shielding the circuit is attached on a main surface 21b opposite to a main surface 21a on which the antenna element 20 is disposed. The signal line 16 is drawn outside through a notch portion formed in the top cover 11.
The antenna device 10 is united in one body by assembling the top cover 11 and the bottom plate 12 with the antenna module 14 and the packing member 15 housed in the inner space of the top cover 11. The packing member 15 is made of a resin material such as an EPDM rubber.
When the top cover 11 and the bottom plate 12 are joined, the packing member 15 is interposed therebetween in order to ensure sealing-ability of the junction portion. A gasket 15b rises from a base 15a corresponding to the notch portion 11a of the top cover 11.
A single concave portion 12a is formed on the center of the bottom plate 12 and a permanent magnet 30 is disposed in the concave portion 12a. The permanent magnet 30 is disposed in order to attach the antenna device 10 on the roof of the vehicle. Further, on the outer main surface of the bottom plate 12, a resin sheet 31 for preventing a damage of the vehicle is attached over the substantially entire surface of the main surface. A product number or name of the antenna device 10 is printed on the resin sheet 31.
In the related-art antenna device having the above-mentioned structure, an antenna device in which a tuner unit is mounted has been put to practical use. In this type of the antenna device, the size of the circuit board 21 increases since the tuner unit is mounted. Consequently, the entire size of the antenna device is larger.